At present, batch processes are applied to the production of dextrose by saccharification of starch hydrolyzates with a glucoamylase. Many of the commercially-available glucoamylase enzymes produced today are derived from microorganisms of the genera Rhizopus and Aspergillus. When these enzymes are used to produce dextrose, they are generally reacted at 55.degree. C. to 60.degree. C. for 2 to 4 days. If glucoamylase can be immobilized and the saccharification can be conducted continuously through a column, the reaction time is reduced and no large reaction tank is necessary thereby saving labor and energy. Glucoamylase enzymes produced by Rhizopus and Aspergillus can be immobilized by ion exchange processes, physical adsorption, covalent bonding, gel entrapment, etc. Any glucoamylase enzymes immobilized by any of these processes are, however, inactivated when used at above 50.degree. C.
An article by Marsh, D. R., Lee, Y. Y., and Tsao, G. T., Biotech. Bioeng. 15, 483 (1973) reported that immobilized glucoamylase enzymes were stable for a considerably long period of time when they were used at below 50.degree. C., but this process is not commercially feasible because of the danger of microbial contamination.
Therefore, the development of an immobilized glucoamylase stable at temperatures above 50.degree. C. is necessary for continuous saccharification in commercial operations.
To achieve this, a glucoamylase having remarkably higher thermostability than those of conventional ones must be developed.